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Pt_xIr_100–x thin films (8 nm thick) were deposited on MgO(100) substrates by pulsed laser deposition at 600 °C. As shown by X-ray diffraction and X-ray photoelectron spectroscopy analyses, the formation of fcc bulk and surface PtIr solid solution was observed over the whole compositional range despite the large miscibility gap exhibited by these elements below ca. 1000 °C. Moreover, pole figures indicate that all films exhibit a Pt(001)[010]//[010](001)MgO cube-on-cube epitaxial relationship, which is consistent with AFM images. Cyclic voltammetry measurements performed in alkaline solutions confirmed both the presence of Ir atoms at the surface and the (100) surface orientation of all Pt_xIr_100–x surfaces. The highest electrocatalytic activity for the electrooxidation of NH₃ was observed for the Pt₇₄Ir₂₆(100) surface with a current density of ca. 1.0 mA cm^–2 at 0.71 V vs RHE, which is 25% higher than that on Pt …